1. Field of the Invention
The present invention relates to a pressure or pressurized oil supply/discharge circuit with a residual pressure holding function and a residual pressure holding valve device for use in the pressurized oil supply/discharge circuit, which are adapted to maintain a predetermined pressure within a hydraulic actuation chamber even in the event that a breakage of a hydraulic hose and the like causes on oil leak in a pressurized oil supply/discharge passage connected to the hydraulic actuation chamber of a hydraulic cylinder.
2. Description of the Prior Art
A known pressure oil supply/discharge circuit with a residual pressure holding function has been disclosed in Japanese Patent Laid Open Publication No. 1979-108156.
As shown in the circuit diagram of FIG. 21, this publication discloses a hydraulic cylinder applied to a clamping device.
That is, a clamping hydraulic actuation chamber 403 is disposed on one side of a piston 402 of a double-acting type hydraulic cylinder 401, a check valve 405 with a forcibly valve-opening (actuator) function is disposed in a clamping pressure oil supply/discharge passage 404 connected to the hydraulic actuation chamber 403, and an unclamping pressure oil supply/discharge passage 407 is connected to an unclamping actuation chamber 406 formed on the other side of the piston 402.
At the time of clamping, oil is discharged from the unclamping hydraulic actuation chamber 406 and the clamping pressure oil is supplied to the clamping hydraulic actuation chamber 403, so that the hydraulic cylinder 401 is operatively contracted to press and fix a fixed object 410 by means of a clamping member 409. Under this clamping condition, when a pressure on the inlet side of the check valve 405 abnormally lowers due to a breakage of a hydraulic hose and the like, the check valve 405 serves to prevent a counter flow so as to leave or maintain a pressure remaining within the clamping hydraulic actuation chamber 403 and thus to prevent a shift and/or a fall of the fixed object 410. Incidentally, slight leakage of pressure from the check valve 405 is adapted to be supplemented by means of an accumulator 411.
On one hand, at the time of unclamping, when the pressurized oil is supplied from the unclamping pressure oil supply/discharge passage 407 to the unclamping hydraulic actuation chamber 406, the check valve 405 is opened by means of hydraulic pressure via a pilot oil passage 413, so that pressurized oil is allowed to be discharged from the clamping hydraulic actuation chamber 403 and the hydraulic cylinder 401 is operatively extended.
The above-mentioned prior art has the advantage that the clamping condition can be held by means of a one-way checking function of the check valve 405 even though a pressure oil leakage is caused on the inlet side of the check valve 405, but there is the following problem associated therewith.
For example, like in a hydraulic clamping device for a metal mould of an injection moulding machine, when the hydraulic cylinder is subjected to a raised temperature under the clamping condition, pressure within the clamping hydraulic actuation chamber 403 gradually increases due to a cubical (volumetric) expansion of the pressurized oil. Thereby, the clamping force of the hydraulic cylinder 401 may increase sufficiently to give concern over damage to a fixed object 410.